Method for measuring tourniquet limb occlusion pressure

ABSTRACT

Improved tourniquet apparatus for measuring a patient&#39;s limb occlusion pressure includes an inflatable tourniquet cuff for encircling a limb at a location and to which a tourniquet instrument is releasably connectable. The instrument includes pressure sensing for producing a cuff pressure signal indicative of the level of pressure in the cuff, a pressure regulation mechanism communicating with the cuff and responsive to the cuff pressure signal for moving fluid into and out of the cuff, thereby regulating the pressure in the cuff, a blood flow transducer for producing a blood flow signal indicative of blood flow past the cuff, and limb occlusion pressure means responsive to the blood flow signal and the cuff pressure signal for increasing the cuff pressure level until blood flow indicated by the blood flow signal decreases to a level less than a minimum detection threshold, and then suspending fluid movement into and out of the cuff to produce a limb occlusion pressure value that is indicative of a pressure in the cuff when the fluid motion is suspended.

FIELD OF THE INVENTION

This invention pertains to pneumatic tourniquet systems commonly usedfor stopping the flow of arterial blood into a portion of a surgicalpatient's limb to facilitate the performance of a surgical procedure,and for facilitating intravenous regional anesthesia. In particular,this invention pertains to pneumatic tourniquet apparatus for measuringthe minimum pressure that must be applied to stop arterial blood flowinto the portion of the limb to facilitate surgery.

BACKGROUND OF THE INVENTION

Surgical tourniquet systems are commonly used to stop the flow ofarterial blood into a portion of a patient's limb, thus creating aclear, dry surgical field that facilitates the performance of a surgicalprocedure and improves outcomes. A typical surgical tourniquet system ofthe prior art includes a tourniquet cuff for encircling a patient's limbat a desired location, a tourniquet instrument, and flexible tubingconnecting the cuff to the instrument. In some surgical tourniquetsystems of the prior art, the tourniquet cuff includes an inflatableportion, and the inflatable portion of the cuff is connectedpneumatically through one or two cuff ports by flexible plastic tubingto a tourniquet instrument that includes a pressure regulator tomaintain the pressure in the inflatable portion of the cuff, whenapplied to a patient's limb at a desired location, near a referencepressure that is above a minimum pressure required to stop arterialblood flow past the cuff during a time period suitably long for theperformance of a surgical procedure. Many types of such pneumaticsurgical tourniquet systems have been described in the prior art, suchas those described by McEwen in U.S. Pat. No. 4,469,099, U.S. Pat. No.4,479,494, U.S. Pat. No. 5,439,477 and by McEwen and Jameson in U.S.Pat. No. 5,556,415 and U.S. Pat. No. 5,855,589.

Some advanced tourniquet systems include tourniquet cuffs that have twoseparate pneumatic cuff ports, so that two separate pneumaticpassageways can be established between the inflatable portion of thecuff and the tourniquet instrument, by separately connecting flexibleplastic tubing between each port and the instrument. Such systems areoften called dual-port tourniquet systems. In one such dual-porttourniquet system of the prior art, described in U.S. Pat. No.4,469,099, the pneumatic pressure regulation elements within thetourniquet instrument communicate pneumatically with the inflatableportion of the cuff through one port, and a pressure sensor within thetourniquet instrument communicates pneumatically with the inflatableportion of the cuff through the second port. This configuration enablesmore accurate sensing, monitoring and continuous regulation of theactual pressure in the inflatable portion of the cuff that encircles thepatient's limb, in comparison to single-port tourniquet systems.

In a typical single-port tourniquet system of the prior art, thetourniquet cuff has only one port and only one pneumatic passageway isestablished between the tourniquet cuff and the instrument. The actualcuff pressure must be sensed indirectly, through the same tubing andport that is used to increase, decrease and regulate the pressure in thecuff during surgery. As a result, in such a single-port tourniquetsystem of the prior art, the accuracy and speed of pressure regulation,and the accuracy of the sensed cuff pressure, are affected by thepneumatic flow resistance within the single port and within the flexibleplastic tubing that pneumatically connects the port and cuff to thetourniquet instrument. These characteristics inherent in single-porttourniquet systems may also affect the accuracy and speed of measurementof Limb Occlusion Pressure (LOP, defined below), in comparison todual-port tourniquet systems.

Many studies published in the medical literature have shown that thesafest tourniquet pressure is the lowest pressure that will stop theflow of arterial blood past a specific cuff applied to a specificpatient for the duration of that patient's surgery. Such studies haveshown that higher tourniquet pressures are associated with higher risksof tourniquet-related injuries to the patient. Therefore, when atourniquet is used in surgery, surgical staff generally try to use thelowest tourniquet pressure that in their judgment is safely possible.

It is well established in the medical literature that the optimalguideline for setting the pressure of a constant-pressure tourniquet isbased on Limb Occlusion Pressure (LOP). LOP can be defined as theminimum pressure required, at a specific time in a specific tourniquetcuff applied to a specific patient's limb at a specific location, tostop the flow of arterial blood into the limb distal to the cuff. Thecurrently established guideline for setting tourniquet pressure based onLOP is that an additional safety margin of pressure is added to themeasured LOP, to account for physiologic variations and other changesthat may be anticipated to occur normally over the duration of asurgical procedure.

Surgical staff can measure LOP manually by detecting the presence ofarterial pulsations in the limb distal to a tourniquet cuff as anindicator of arterial blood flow past the cuff and into the distal limb.Such arterial pulsations can be defined as the rhythmical dilation orthrobbing of arteries in the limb distal to the cuff due to blood flowproduced by regular contractions of the heart. Detecting blood flow thuscan be done using palpation, Doppler ultrasound or photoplethysmographyto measure arterial pulsations. One technique for manual measurement ofLOP based on monitoring arterial pulsations as an indicator of arterialblood flow is as follows: tourniquet cuff pressure is increased by anoperator slowly from zero while monitoring arterial pulsations in thelimb distal to the cuff until the pulsations can no longer be detected;the lowest tourniquet cuff pressure at which the pulsations can nolonger be detected can be defined as the ascending LOP. A second manualtechnique is that an operator can slowly decrease tourniquet cuffpressure while monitoring to detect the appearance of arterialpulsations distal to the cuff; the highest pressure at which arterialpulsations are detected can be defined as the descending LOP. Theaccuracy of such manual measurements of LOP is very dependent on thesensitivity, precision and noise immunity of the technique for detectingand monitoring arterial pulsations, and on operator skill, technique andconsistency. Under the best circumstances considerable elapsed time isrequired on the part of a skilled, experienced and consistent operator,using a sensitive and precise technique for detecting and monitoringpulsations as an indicator of distal blood flow, to accurately measureLOP by manual means.

Some surgical tourniquet systems of the prior art include means tomeasure LOP automatically. Prior-art tourniquet apparatus havingautomatic LOP measurement means are described by McEwen in U.S. Pat. No.5,439,477, by McEwen and Jameson in U.S. Pat. No. 5,556,415 and byMcEwen et al in co-pending US Patent Application Publication No.20060253150. Such prior-art systems have included blood flow transducersthat employ a photoplethysmographic principle to sense blood flow in thedistal limb, although other transducers have been suggested in the priorart to measure blood flow based on other principles. A blood flowtransducer employing the photoplethysmographic principle uses light toindicate the volume of blood present in a transduced region, consistingof a combination of a residual blood volume and a changing blood volumeresulting from arterial pulsations. An additional pressure margin basedon recommendations in published surgical literature is added to theautomatically measured LOP to provide a “Recommended TourniquetPressure” (RTP), as a guideline to help the surgical staff select thelowest tourniquet pressure that will safely stop arterial blood flow forthe duration of a surgical procedure. Such prior-art systems allow thesurgical staff to select the RTP, based on LOP, as the tourniquetpressure for that patient or to select another pressure based on thephysician's discretion or the protocol at the institution where thesurgery is being performed. The difference in pressure between themeasured LOP and the tourniquet pressure selected for surgery, which maybe the RTP, can be defined as the cuff pressure safety margin. Ideallythe cuff pressure safety margin is selected to be greater than themagnitude of any increase in LOP normally expected during surgery due tochanges caused by drugs used for anesthesia, the patient's physiologicresponse to surgery and other variables. Change in blood pressure is onephysiologic characteristic that varies during surgery and has been shownto affect the LOP during surgery, and therefore the cuff pressure safetymargin during surgery. For example, an increase in the patient's bloodpressure will lead to an increase in LOP, with attendant decrease in thesafety margin.

Despite their potential to recommend near-optimal settings of surgicaltourniquet pressures for individual patients, some prior-art surgicaltourniquet systems that include means for automatic measurement of LOPhave demonstrated limitations of performance that have prevented theirwidespread acceptance and routine use. The limitations are primarily infour areas: safety, probability of successful LOP measurement, speed ofLOP measurement, and accuracy of LOP measurement.

Regarding safety, it is desirable during LOP measurement that thetourniquet cuff pressure not rise significantly above the pressurerequired to stop blood flow past the cuff for a significant period oftime. This is because it is well established that the possibility oftourniquet-related injuries increases if tourniquet cuff pressureincreases substantially. For this reason, prior-art tourniquet apparatusthat measures LOP by descending from a high cuff pressure are consideredto be less desirable than tourniquet apparatus that measures LOP byascending from a low pressure. Also regarding safety, it is desirablethat LOP measurements be made as quickly as possible, while stillassuring that the resulting LOP measurement is sufficiently accurate toallow setting the tourniquet pressure based on the measured LOP. Speedof LOP measurement is desirable for three reasons related to safety andperformance: first, it is well established that longer tourniquet timesare associated with a higher possibility of tourniquet-related injuries;second, during LOP measurement, if venous outflow of blood from the limbis restricted by a pressurized tourniquet cuff for an excessively longperiod of time, then pooling of blood in the distal limb from arterialinflow may occur, possibly leading to passive congestion of the limbfrom residual blood that may be hazardous; and third, any continuingincrease of residual blood in the distal limb over an extendedmeasurement period may lead to measurement error inphotoplethysmographic blood flow transducers, because such transducersinherently provide one indication of the combination of residual bloodvolume and varying blood volume resulting from arterial pulsations inthe transduced portion, thus lengthening the time for successfulcompletion of LOP measurement, or making successful LOP measurementimpossible.

Experience with manual LOP measurement, and with prior-art tourniquetapparatus having LOP measurement capability, has shown that it is notpossible in practice to measure the LOP of all patients. This is becausethe quality and magnitude of arterial blood flow measured by a bloodflow transducer distal to the tourniquet cuff may not be sufficient insome patients for measurement or analysis, due to a variety of anatomicand physiologic factors. For such patients, the physician must revert toa standard tourniquet pressure setting based on the physician'sdiscretion. Co-pending US Patent Application Publication No. 20060253150describes means for characterizing some aspects of the quality of bloodflow distal to the tourniquet cuff measured by a blood flow transducer,in order to quickly identify some patients and situations in which LOPmeasurement is unlikely to be successfully completed. No tourniquetsystem known in the prior art provides a blood flow transducer having anindication of the quality of blood flow at the transducer that isperceptible to an operator, to assist the operator in positioning andadjusting the blood flow transducer on the limb to improve the qualityof measured blood flow prior to initiation of LOP measurement.

Even for patients in whom LOP measurement is possible, the time requiredby some tourniquet systems known in the prior art to successfullycomplete automatic LOP measurements may be considerable. In addition tothe safety-related considerations described above, the extended timerequired for LOP measurement by some prior-art tourniquet systems maysignificantly disrupt or delay normal activities in the operating room,and thus affect the efficiency of surgery. This is in part because thepatient's operative limb must remain motionless during the measurementperiod, to avoid the introduction of variations in pneumatic cuffpressure and the introduction of noise due to movement of the distalblood flow transducer relative to the limb. In some prior-art apparatusfor measuring LOP, the reference pressure for the tourniquet cuff istypically increased from zero in many predetermined increments ofincreasing pressure. After each such predetermined increment or step ofthe reference pressure, time is required to allow the actual increasedpressure within the tourniquet cuff to stabilize before measurements canbe taken from the distal blood flow transducer and related to actualcuff pressure. For single-port tourniquet systems of the prior art, thetime required for the cuff pressure to stabilize is significant.Substantially increasing the predetermined step size in such prior-artsystems might increase the speed of LOP determination, but could alsodecrease the accuracy of LOP measurement significantly. Thus the totaltime required for sufficiently accurate LOP measurement in theseprior-art systems can be substantial, and includes the time required toincrease the reference pressure in many predetermined steps from zero,the time required to allow the actual cuff pressure to stabilize aftereach step, and the time required to take a measurement from the distalblood flow transducer at each step, until LOP measurement issuccessfully completed or until an arbitrary maximum pressure limit isreached without LOP being measured.

The accuracy of LOP measurements by prior-art tourniquet apparatus maybe affected by three additional sources of error. First, because of thesubstantial time periods often required to measure LOP by prior-arttourniquet apparatus, error may be introduced into the LOP measurementdue to accumulation of residual blood in the limb distal to thetourniquet cuff. This gradual accumulation of residual blood due toblocking of venous outflow by the tourniquet cuff can reduce themagnitude of the pulsations in blood volume that are associated with therhythmical dilation or throbbing of the distal arteries over theduration of each cardiac cycle, from heartbeat to heartbeat. Also, suchan increasing volume of residual blood in the distal limb during ameasurement interval can cause a gradual change in the mean blood flowsignal from a photoplethysmographic transducer during the period, forreasons described above. Such a gradual change may make valid arterialpulsations indicating arterial blood flow difficult or impossible todetect, and reduces the maximum possible amplification of the signalfrom the distal blood flow transducer, thus reducing the accuracy ofsubsequent analysis. A second source of error in LOP measurement by someprior-art tourniquet apparatus results from movement of the patient'slimb and movement of the distal blood flow transducer relative to theattached limb, either of which could mask valid arterial pulsationsindicating blood flow or could be misinterpreted as valid arterialpulsations. A third source of error in LOP measurement by prior-arttourniquet apparatus results from improper application and use of thephotoplethysmographic transducer. For example, the transducer may be notbe applied properly to the digit of a limb by the operator, or thetransducer may be applied so that it is exposed to the directillumination of surgical lighting.

There is a need for improved surgical tourniquet apparatus for measuringLOP, to overcome the above-described limitations of prior-art tourniquetsystems, so that such apparatus will be suitable for routine use in allsurgical procedures. To be routinely useful in this context, apparatusfor measuring LOP automatically should not introduce secondary hazardsassociated with the measurement of LOP, should indicate to an operatorwhether an LOP measurement is possible prior to initiation, should havea high probability of successful completion after LOP measurement isinitiated, should complete LOP measurement sufficiently fast so that themeasurement of LOP does not disrupt or unduly delay normal activities inthe operating room, should result in an LOP measurement that is accuratewithin surgically acceptable expectations so that it can be used as thebasis for optimal setting of tourniquet pressure. Also, after initialsetting of tourniquet pressure, it would be desirable to provide anoperator with an ongoing indication of the tourniquet cuff pressuresafety margin during surgery. Certain improvements to tourniquetapparatus for measuring LOP have been described in co-pending US PatentApplication No. 20060253150. The present invention further addresses theneed for improved tourniquet apparatus for measuring LOP.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pictorial representation of the preferred embodiment in asurgical application.

FIG. 2 is a block diagram of the preferred embodiment.

FIG. 3 is an illustration that shows increases in the level of the cuffreference pressure in synchrony with arterial pulsations detected duringthe measurement of limb occlusion pressure by the preferred embodiment.

FIG. 4 is a flow chart depicting the sequence of operations performed toevaluate blood flow signal quality.

FIG. 5 and FIG. 6 are flow charts depicting the sequence of operationsperformed by the preferred embodiment during measurement of limbocclusion pressure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiment illustrated is not intended to be exhaustive or limit theinvention to the precise form disclosed. It is chosen and described inorder to explain the principles of the invention and its application andpractical use, and thereby enable others skilled in the art to utilizethe invention.

Hardware

FIG. 1 shows blood flow transducer 2 applied to a digit of patient limb4 and connected to instrument 6 via multi-conductor shielded cable 8.Blood flow transducer 2 is positioned on patient limb 4 at a locationthat is distal to pressurizing cuff 10 which is also shown applied topatient limb 4. This configuration permits blood flow transducer 2 todetect blood flow in patient limb 4 and changes in blood flow that occurin patient limb 4 as a result of the pressurization of cuff 10. Bloodflow transducer 2 is used by instrument 6 when instrument 6 isperforming automatic measurements of limb occlusion pressure (LOP). LOPhas been defined above to be the minimum pressure required, at aspecific time in a specific tourniquet cuff applied to a specificpatient's limb at a specific location, to stop the flow of arterialblood into the limb distal to the cuff.

Blood flow signal quality indicator 12 forms part of blood flowtransducer 2 and is configured such that it is readily visible to anoperator of transducer 2. In the preferred embodiment blood flow signalquality indicator 12 is a bi-color green and red LED, but it will beapparent that other types of indicators visible to the operator oftransducer 2 could be employed. The green LED of blood flow signalquality indicator 12 is illuminated to provide an indication of whensignals representative of blood flow detected by transducer 2 exceedpredetermined minimum quality criteria and an automatic measurement oflimb occlusion pressure may proceed. If the signals representative ofblood flow detected by transducer 2 do not exceed predetermined minimumquality criteria, the red LED of blood flow signal quality indicator 12is illuminated and the automatic measurement of limb occlusion pressureis inhibited until the operator repositions the transducer or makesother adjustments to improve signal quality. The immediate feedback ofblood flow signal quality at the location of blood flow transducer 2provides the operator with a convenient means to be assured that bloodflow transducer 2 has been correctly applied to a digit of a patientlimb 4, a blood flow signal of acceptable quality is available, and thatan automatic measurement of LOP is possible.

Blood flow transducer 2 also includes LOP measurement key 14, which whendepressed will initiate the automatic measurement of limb occlusionpressure.

Cuff 10 is pneumatically connectable to instrument 6. The inflatableportion of pressurizing cuff 10 has one pneumatic connection and isgenerally similar in design and construction to the cuffs described byMcEwen in U.S. Pat. No. 5,741,295, U.S. Pat. No. 5,649,954, U.S. Pat.No. 5,484,831 and by Robinette-Lehman in U.S. Pat. No. 4,635,635. Cuff10 is adapted for use in a sterile surgical field in an operating roomenvironment by being formed of materials that can withstand, and thatcan be sterilized by, techniques normally used to sterilize medicaldevices to a level of sterility that allows them to be safely usedwithin a sterile surgical field. Cuff 10 is a single-port cuff, apneumatic passageway to the inflatable portion of cuff 10 is provided bycuff port 16. In FIG. 1 cuff port 16 of sufficient length to allow apneumatic connection to cuff 10 to be made outside of a sterile surgicalfield. Cuff port 16 is fitted with male locking connector 18 (DSM2202,Colder Products Company, St. Paul, Minn.), and mate to form releasablepneumatic connection with female locking connector 20 (PMC1704, ColderProducts Company, St. Paul, Minn.). For clarity, the connectorsillustrated in FIG. 1 are shown disconnected; in the followingdescription of the preferred embodiment the connectors are mated andform part of the pneumatic passageway between instrument 6 and cuff 10.The pneumatic connection from instrument 6 to cuff 10 is made byflexible plastic tubing 22 which is fitted with female locking connector20.

As can be seen in FIG. 1, instrument 6 includes an operator interfaceconsisting of graphic display panel 24, keypad 26, and visual alarmindicator 28. Display panel 24 is employed for the selective display ofany of the following alphanumeric information: limb occlusion pressures,recommended tourniquet pressures and cuff pressure safety margins asmeasured and updated by instrument 6; actual cuff pressures as measuredby instrument 6; reference or “set” cuff pressure levels, alarmreference “limits” or values; alphanumeric alarm messages describingdetected alarm conditions and other information required for theoperation of instrument 6.

Keypad 26 provides a means for an operator of instrument 6 to controlthe operation of instrument 6. Keypad 26 includes a limb occlusionpressure measurement (LOP) key 30, which when depressed will initiatethe measurement of LOP as described further below. Keypad 26 also has an“inflate” key to initiate the inflation of cuff 10, a “deflate” key toinitiate the deflation of cuff 10, and other keys to permit the operatorof instrument 6 to adjust the reference pressure level and set inflationtime and other alarm limits.

Visual alarm indicator 28 is a bright red light emitting diode (LED)which is activated by instrument 6 in response to detected alarmconditions. Instrument 6 also signals the presence of an alarm conditionby generating an audible tone to further alert the operator to thepresence of an alarm condition and displays alarm text messagesdescribing the alarm condition on display panel 24. One example of adetected alarm condition that requires the operator's attention is achange in the cuff pressure safety margin as described elsewhere below.

Referring to the block diagram of instrument 6 shown in FIG. 2,controller 32 comprises a microcontroller (MC68HC16Z1, FreescaleSemiconductor, Austin, Tex.), associated memory and control software,analog and digital peripheral interface circuitry, and other necessarysupport components.

As shown in FIG. 2, pneumatic pump 34 (KNF Neuberger, Inc., Trenton,N.J.) is pneumatically connected to reservoir 36 by tubing 38. Inresponse to control signals from controller 32, pump 34 operates topressurize reservoir 36. Reservoir pressure transducer 40 ispneumatically connected by tubing 42 to reservoir 36 and generates areservoir pressure signal. The reservoir pressure signal is communicatedto controller 32. Controller 32 acts to maintain the pressure inreservoir 36 near a reservoir pressure level. Controller 32 sets thereservoir pressure level to a pressure above the reference pressurelevel set by the operator of instrument 6 or automatically by controller32 during a limb occlusion pressure measurement; the reservoir pressurelevel is set to a level significantly greater than the referencepressure level, typically 100 mmHg Controller 32 in response to thereservoir pressure level and the reservoir pressure signal activatespump 34 to maintain the level of the reservoir pressure signal near thereservoir pressure level.

Inflation valve 44 (EVO-3-12V, Clippard Instrument Laboratory,Cincinnati, Ohio) is configured as a two position normally closed valve.One side of the valve is pneumatically connected via tubing 46 toreservoir 36 the other side of the valve is connected to cuff 10 via thepneumatic passageway formed by manifold 48, tubing 22, connectors 20 and18 and cuff port 16. When energized by controller 32, inflation valve 44moves to the open position and allows pressurized gas to flow fromreservoir 36 to cuff 10, thereby increasing the pressure of gas in theinflatable portion of cuff 10.

Deflation valve 50 (EVO-3-12V, Clippard Instrument Laboratory,Cincinnati, Ohio) is configured as a two position normally closed valve.One side of the valve is pneumatically connected to cuff 10 via thepneumatic passageway formed by manifold 48, tubing 22, connectors 20 and18 and cuff port 16, the other side is open to atmosphere. Whenenergized by controller 32, deflation valve 50 moves to the openposition and allows pressurized gas to flow from cuff 10 to atmosphere,thereby decreasing the pressure of gas in the inflatable portion of cuff10.

Cuff pressure transducer 52 is pneumatically connected to cuff 10 viamanifold 48 and the pneumatic passageway formed by tubing 22, connectors20 and 18 and cuff port 16 and generates a cuff pressure signal which iscommunicated to controller 32. Controller 32 is able to resolve changesin the cuff pressure signal as small as 0.15 mmHg. An accuratemeasurement of the actual pressure of gas within cuff 10 by cuffpressure transducer 52 is possible when deflation valve 50 and inflationvalve 44 are both closed because there is no gas flow in the pneumaticpassageways connecting transducer 52 to the inflatable portion of cuff10. To enable accurate cuff pressure measurements during the measurementof Limb Occlusion Pressure, deflation valve 50 and inflation valve 44are both temporarily closed at selected times, thereby suspendingpressure regulation during those times as described further below. Asnoted above, controller 32 will, in response to generated alarm signalsalert the operator of an alarm condition by activating visual alarmindicator 28 and producing audible tones. Speaker 54 is connected tocontroller 32, and electrical signals having different frequencies tospecify different alarm signals and conditions are produced bycontroller 32 and converted to audible sound by loudspeaker 54.

During surgery a patient's physiologic status is monitored by means of apatient monitor. Physiologic characteristics that are typicallymonitored include blood pressure values resulting from periodicnon-invasive or continuous blood pressure measurements, heart ratevalues, temperature values, oxygen saturation and other parameters. Asshown in FIG. 2 instrument 6 includes a physiologic characteristicacquisition module 56 for communication with an external patientmonitor. Physiologic characteristic acquisition module 56 is electroniccircuitry and software that is configured for communicating with thedata communication interface of an external patient monitor to acquirethe values of monitored physiologic characteristics such as bloodpressure. As described further below, to allow an updated cuff pressuresafety margin to be computed and displayed, controller 32 viaphysiologic characteristic acquisition module 56 requests and receivesthe current values of the patient's blood pressure near the time of LOPmeasurement and subsequently while cuff 10 is pressurized to occludeblood flow.

Power supply 58 connects to an external AC supply and provides regulatedDC power for the normal operation of all electronic components ofinstrument 6. Power supply 58 may also include a battery to enableinstrument 6 to continue to operate in the absence of an external ACsupply.

Pressure Regulation

An operator of instrument 6 may use keypad 26 to select a referencepressure level; this is the pressure of gas that instrument 6 willattempt to maintain in the inflatable portion of cuff 10 when cuff 10 isinflated. Controller 32 will generate high or low pressure alarm signalsif the pressure in cuff 10 cannot be maintained near the selectedreference pressure level. If the cuff pressure level exceeds thereference pressure level by 15 mmHg a high pressure alarm signal will begenerated by controller 32. If the cuff pressure level falls below thereference pressure level by 15 mmHg a low pressure alarm signal will begenerated by controller 32.

When controller 32 detects that the “inflate” key on keypad 26 has beendepressed by the operator of instrument 6, controller 32 operates toinflate cuff 10 to a pressure near the selected reference pressure leveland to then regulate the pressure in cuff 10 near the reference pressurelevel until such time that controller 32 detects that the “deflate” keyon keypad 26 has been depressed by the operator of instrument 6.Controller 32 may also inflate, adjust the reference pressure level, anddeflate cuff 10 automatically during a limb occlusion pressuremeasurement as described further below.

To inflate and regulate the pressure in cuff 10 controller 32 includes apressure regulator; the pressure regulator in the preferred embodimentis implemented as a control algorithm that operates as described below.At regular predetermined regulation intervals of 40 ms controller 32computes a pressure error signal. The pressure error signal correspondsto the difference between the reference pressure level and the cuffpressure level. Controller 32 uses the pressure error signal as a termin a proportional integral control algorithm to calculate activationtime intervals for inflation valve 44 and deflation valve 50. Toincrease the gas pressure in cuff 10 when the cuff pressure signal isbelow the reference pressure level, the activation time interval fordeflation valve 50 is set to zero and the activation time interval forinflation valve 44 is proportional to the magnitude of the pressureerror signal and the integral of the pressure error signal. To decreasethe gas pressure in cuff 10 when the cuff pressure signal is above thereference pressure level, the activation time interval for inflationvalve 44 is set to zero and the activation time interval for deflationvalve 50 is proportional to the magnitude of the pressure error signaland the integral of the pressure error signal. Controller 32 limits themaximum valve activation time intervals of valve 44 and valve 50 to theregulation interval time (40 ms). It will be appreciated by thoseskilled in the art that alternate pressure regulation algorithms couldbe employed to control the activation of inflation valve 44 anddeflation valve 50 in response to a cuff pressure signal and a referencepressure level, or that proportional valves could be used instead of thevalves used in the preferred embodiment. Also it will be appreciatedthat a regulator has a response time, consisting of the amount of timerequired for the pressure of gas in the cuff to reach the level of thereference pressure level after a new reference pressure level has beenselected. The regulator response time will depend upon the magnitude ofthe change in reference pressure level, the volume of cuff 10 and thecharacteristics of the pneumatic components in instrument 6 and thespecifics of the control algorithm used. Thus the actual pressure of gasin cuff 10 may differ substantially from the reference pressure levelfor a varying period of time after a change in the reference pressurelevel.

In order to correctly regulate the pressure of gas in cuff 10 at apressure near the cuff pressure reference level and correctly indicateover and under pressure alarm conditions, controller 32 must haveavailable an indication of the pressure within the inflatable portion ofcuff 10. In the preferred embodiment the measurement of the pressure ofgas in cuff 10 is facilitated by cuff pressure transducer 52 and thedirect pneumatic connection between the inflatable portion of cuff 10and transducer 52. Gas flow in the pneumatic passageway connecting cuffpressure transducer 52 with the inflatable portion of cuff 10 caused bythe opening of inflation valve 44 and deflation valve 50 produces errorin the cuff pressure measurement made by pressure transducer 52. Anaccurate measurement of the pressure of gas in cuff 10 is critical tothe ability of instrument 6 to accurately and rapidly measure LOP, asexplained below. During a measurement of LOP, and upon the detection ofan arterial pulsation in blood flow, controller 32 closes deflationvalve 50 and inflation valve 44 to suspend pressure regulation for apredetermined time period (typically 30 ms); this temporary suspensionof pressure regulation in a single-port tourniquet system enables anaccurate measurement of cuff pressure near the time that the arterialpulsation occurred. In a representative implementation as depicted inFIG. 3, the temporary suspension of pressure is shown at intervals 319A,319B and 319C on a curve 312 representing the cuff pressure.

Blood Flow Transducer and Signal Processing

Referring again to FIG. 2, the internal components of blood flowtransducer 2 are shown in detail. Blood flow transducer 2 of thepreferred embodiment employs the principle of photoplethysmography andis adapted for positioning on the limb distal to the tourniquet cuff,although it will be appreciated that other types of blood flowtransducers employing other principles may be used, and it will beappreciated that some types of blood flow transducers may be physicallyintegrated into the structure of a tourniquet cuff. In the preferredembodiment, blood flow transducer 2 has a hinged plastic housing that isconfigured for application to a digit of a limb. Blood flow transducer 2may be applied to a finger or thumb of the hand or a toe of the foot.Transducer 2 includes an infrared light emitting diode (IRLED) 60 and aphotodiode 62 which is sensitive to the wavelength of light emitted byIRLED 60. In the preferred embodiment an IRLED with a wavelength of 915nm is employed. Within blood flow transducer 2 IRLED 60 and photodiode62 are positioned directly opposite each other such that light emittedby IRLED 60 is readily detected by photodiode 62. When applied to adigit IRLED 60 illuminates a volume of tissue and photodiode 62 detectsthe light that is transmitted through this volume of tissue.

IRLED 60 is connected via multi-conductor cable 8 to adjustable constantcurrent source 64. The intensity of light emitted by IRLED 60 isproportional to the amount of electrical current that flows throughIRLED 60. Controller 32 communicates with adjustable constant currentsource 64 to set the level of current that flows through IRLED 60 andthereby the intensity of light emitted by IRLED 60. In the preferredembodiment the current source 64 can be adjusted to supply electricalcurrent ranging from 0 to 100 milliamps in steps of 0.1 milliamps bycontroller 32.

Photodiode 62 generates an electrical current that is linearlyproportional to the intensity of light that strikes the light sensitivearea of photodiode 62. Photodiode 62 is connected by multi-conductorcable 8 to blood flow signal processor 66. Signal processor 66amplifies, filters, and digitizes the current generated by photodiode 62to produce a blood flow signal that is representative of the intensityof light that strikes photodiode 62. The characteristics of photodiode62 and the electronic circuits within signal processor 66 determine theminimum and maximum light intensities that the blood flow signal canrepresent. As described below, the preferred embodiment operates tomaintain the level of the blood flow signal within the dynamic range ofsignal processor 66.

When blood flow transducer 2 is applied to a digit of a patient's limbthe intensity of light reaching photodiode 62 is dependent upon a numberof factors. These factors are the initial intensity of the light emittedby IRLED 60; the amount of light absorbed by the skin pigmentation,tissue and bone of the digit; the amount of light absorbed by venousblood and non-pulsatile arterial blood and pulsatile arterial blood; andthe optical path length between IRLED 60 and photodiode 62. When cuff 10is deflated, a relatively constant amount of light is absorbed by theskin pigmentation, bone other tissue, venous blood and the non-pulsatilepart of the arterial blood. This aggregate non-pulsatile component ofthe blood flow signal, illustrated as non-pulsatile signal 302 in FIG.3, is detected and measured by non-pulsatile level detector 68.Non-pulsatile level detector 68 communicates to controller 32 the levelof non-pulsatile signal 302.

During each cardiac cycle the diameters of the arteries and arteriolesalternately increase and decrease in response to arterial blood flowpulsations. This alternating increase and decrease in diameters affectsthe optical path length between IRLED 60 and photodiode 62 and producesa rhythmical and alternating variation in the intensity of lighttransmitted through the digit that is in synchrony with each cardiaccycle. Typically, this rhythmical and alternating variation of intensityis 1-2 percent of the total amount of light transmitted through thevolume of tissue, and results in the production by signal processor 66of a blood flow signal having alternating variations as illustrated inFIG. 3. Arterial pulsation detector 70 detects an arterial pulsation bydetecting the occurrence of the alternating variation in the blood flowsignal from signal processor 66 that occurs during each cardiac cycle,and further determines the relative magnitude of each detected arterialpulsation by determining the difference between the minimum and maximumof each alternating variation of the blood flow signal, as illustratedin FIG. 3.

FIG. 3 illustrates non-pulsatile signal 302, blood flow signal 304, andarterial pulsations of magnitudes 306, 308 and 310 that decrease as cuffpressure 312 increases in response to increases in reference pressurelevel 314. FIG. 3 also illustrates that cuff pressure 312 may differsignificantly from reference pressure level 314 for varying periods oftime after changes in reference pressure level 314. Finally, FIG. 3illustrates that, in the preferred embodiment, changes in referencepressure level 314 are only made in synchrony with arterial pulsationsdetected by arterial pulsation detector 70, as explained further below.Synchronizing any change in reference pressure level 314 to detectedarterial pulsations is an important characteristic of the preferredembodiment that greatly increases the speed of LOP measurement incomparison to prior-art apparatus in which increases in referencepressure levels are made at arbitrary, unsynchronized times.

The magnitude is affected by the intensity of light emitted by IRLED 60.Generally, as the intensity of light emitted by IRLED 60 increases, thevolume of tissue illuminated by IRLED 60 increases which results in anincrease in the magnitude of the alternating and rhythmical variation ofthe blood flow signal as more arteries and arterioles are illuminated inthe optical path between IRLED 60 and photodiode 62.

The optical path length through the volume of tissue between IRLED 60and photodiode 62 is also affected by any change in diameter of thevenules and the amount of venous blood in the tissue. When cuff 10 ispressurized to a level that is greater than that required to occludevenous blood from flowing out of the limb but still at a level thatallows arterial blood to flow into the limb there is an increase in thevolume of venous blood present in the limb and a corresponding increasein the diameter of the venules. This increase in diameter increases theoptical path length through the volume of tissue and results in adecrease in the amount of light detected by photodiode 62. This decreasein light intensity happens gradually, but may be substantial, resultingin reductions of up to three orders of magnitude of the lighttransmitted through the volume of tissue. In the preferred embodimentthis change in the intensity of light transmitted through the volume oftissue is compensated for by an increase in the intensity of IRLED 60,as described further below. In some circumstances described furtherbelow it may not be possible to compensate for this magnitude of changein intensity as IRLED 60 has an upper limit to the intensity of lightthat it can produce.

Each cardiac cycle that occurs when cuff 10 is at a pressure thatpartially or completely stops venous outflow, but not arterial inflow,results in an increase in the amount of venous blood in the volume oftissue illuminated by IRLED 60. It is important to minimize the timethat cuff 10 is at these pressures because the accumulation of venousblood may be hazardous, as explained above. It is also important thatthis time be minimized to ensure that the photoplethysmographic bloodflow signal remains in a region that is within the dynamic range ofIRLED 60 to illuminate the tissue, and within the dynamic range of theelectronic circuits used to detect and process the signal fromphotodiode 62. The preferred embodiment acts to minimize the time thatcuff 10 is at these pressures when attempting to make a measurement ofLOP by assessing during an initialization period whether such anattempted measurement is likely to be successful, as follows. In theinitialization period, if a blood flow signal cannot be detected bysignal processor 66, or if alternating rhythmical variations of theblood flow signal characterizing arterial pulsations above apredetermined minimum initial magnitude cannot be detected by arterialpulsation detector 70, then controller 32 increases the intensity ofIRLED 60 by adjusting the current to IRLED 60 by means of adjustableconstant current source 68 in an effort to increase the magnitude of theblood flow signal to a level suitable for analysis. If this adjustmentby current source 64 still does not result in a blood flow signal havingvariations greater than the predetermined minimum initial magnitude,then controller 32 promptly terminates the attempt to measure LOP andproduces an appropriate indication perceptible to the operator. In thisway, the preferred embodiment minimizes the duration of an attempt tomeasure LOP that may delay the start of surgery, and that may causevenous blood pooling, if that measurement of LOP is unlikely to besuccessfully completed, and allows the operator to promptly selectanother reference pressure level for the tourniquet system that is notbased on LOP.

If an attempt to measure LOP has not been terminated during theinitialization period, arterial pulsation detector 70 continues toanalyze the blood flow signal from signal processor 66 to detect theoccurrence of each alternating rhythmical variation above a minimumdetection threshold that characterizes an arterial pulsation of bloodflow, and to indicate to controller 32 the magnitude of the differencebetween the maximum and minimum of the alternating rhythmical variation,as illustrated by magnitudes 306, 308 and 310 in FIG. 3. Each magnitudeis representative of the amount of arterial blood flowing into thevolume of tissue between IRLED 60 and photodiode 62 during the period ofeach cardiac cycle. To be correctly identified as an arterial pulsationof blood flow, the magnitude must exceed the minimum detectionthreshold. The minimum detection threshold of arterial pulsationdetector 70 is initially set to a predetermined threshold, and maysubsequently be set by controller 32 to another threshold.

When an arterial pulsation is detected by arterial pulsation detector70, the time of occurrence is communicated to controller 32, andpulsation detector 70 enters a refractory time period immediately afterthe detected occurrence. During the refractory time period, pulsationdetector 70 is non-responsive to the blood flow signal from signalprocessor 66. This non-responsiveness of pulsation detector 70 to theblood flow signal during the refractory time period allows controller 32to make adjustments to the level of the current supplied by adjustableconstant current source 64 to IRLED 60 while preventing pulsationdetector 70 from erroneously analyzing any noise or artifact in theblood flow signal resulting from the adjustments to the level of currentto IRLED 60. During the measurement of LOP, controller 32 typically setsthe refractory time period of pulsation detector 70 to be equal to 75percent of the time between successively detected arterial pulsations.Depending on the time between successive pulsations, the duration of therefractory time period may be adjusted by controller 32 from apredetermined initial time of 350 milliseconds to a predeterminedmaximum time of 1200 milliseconds. As described above, duringmeasurement of LOP, controller 32 suspends pressure regulation inresponse detection of an arterial pulsation and closes deflation valve50 and inflation valve 44 for a time period sufficient for an accuratemeasurement of cuff pressure near the time of the arterial pulsation.

Instrument 6 includes blood flow signal quality indicator 12 to providethe operator with an indication that blood flow transducer 2 has beencorrectly applied to a digit of patient limb 4 and that a blood flowsignal exceeding predetermined minimum quality criteria described belowis being obtained.

As shown in FIG. 2, instrument 6 includes a blood flow signal qualityprocessor 72 that evaluates the blood flow signal quality. For clarity,blood flow signal quality processor 72 has been shown as a separatefunctional block in FIG. 2, although the function performed by bloodflow signal quality processor 72 may be implemented as softwarealgorithms performed by controller 32. Blood flow signal qualityprocessor 72 controls the activation of blood flow signal qualityindicator 12, and enables or inhibits the automatic measurement of LOPin response to the activation of LOP key 14 or LOP key 30.

Blood flow signal quality processor 72 evaluates the blood flow signalquality by continuously monitoring several key characteristics of theblood flow signal that affect the signal quality. Referring to FIG. 4,the evaluation algorithm employed by blood flow signal quality processor72 is shown in detail. The algorithm begins by temporarily adjusting theintensity of light emitted by IRLED 60 to a predetermined minimum level(402), and then determines if blood flow transducer 2 is attached to adigit by monitoring the level of the non-pulsatile signal fromnon-pulsatile level detector 68 (404). If blood flow transducer 2 is notattached to a digit, the level of the non-pulsatile signal will exceed apredetermined threshold due to the saturation of photodiode 62 by lightemitted by IRLED 60; otherwise, the algorithm proceeds to verify thatthe level of ambient light detected is below a predetermined threshold(406).

To determine if the level of ambient light at blood flow transducer 2 isexcessive and may interfere with the measurement of blood flow, bloodflow signal quality processor 72 temporarily switches off IRLED 60, thenacquires the level of the non-pulsatile signal from non-pulsatile leveldetector 68 and then compares the level to a predetermined ambient lightthreshold. Excessive ambient light detected at blood flow transducer 2may be caused by direct illumination from surgical lighting, by poorcontact between transducer 2 and the digit, or by a combination of bothfactors. If the level of ambient light is below the predeterminedambient light threshold, the algorithm proceeds to verify that theamount of light absorbed by the digit is within a predeterminedabsorption tolerance window (408). The light absorption level isdetermined by observing the change in the level of non-pulsatile signalresulting from a variation in the electrical current supplied to IRLED60 and thereby the intensity of IRLED 60. In a digit with normal lightabsorption characteristics, a change in the level of non-pulsatilesignal is proportional to the change in electrical current supplied toIRLED 60. Factors that may affect the light absorption characteristicsof a digit include skin pigmentation, volume and biological structure.For example, if the patient has a thick digit, the light absorbed mayexceed a predetermined maximum absorption level, and thus potentiallyaffect the detection of pulsatile signals during the measurement of LOP.Other non-patient related causes of suboptimal non-pulsatile signallevel may include poor coupling between the surface of the digit andIRLED 60 and photodiode 62, excessive preparation solution obstructingIRLED 60 or photodiode 62, poor positioning of blood flow transducer 2,and poor cleaning of blood flow transducer 2 before use.

The algorithm next verifies that the noise level of the blood flowsignal is below a predetermined noise threshold (410). In the preferredembodiment this is done by detecting the number of zero-crossings andslope changes that occur in the blood flow signal over a predeterminedtime period, however it will be appreciated that other methods may beused to quantify signal noise. In an intraoperative environment, sourcesof noise may originate from the patient having a cold digit that leadsto low perfusion, movement of the patient such as shivering, activitiesfrom surgical staff preparing the limb, and electrical noise from otherequipment in the operating room that may interfere with blood flowtransducer 2.

Next, the algorithm determines if an arterial pulsation having apredetermined minimum magnitude can be detected within a predetermineddetection time window (412). A low magnitude arterial pulsation can becaused by poor perfusion, a cold digit, limb elevation, and improperpositioning of blood flow transducer 2 on the digit. If the magnitude ofthe detected arterial pulsation is greater than the predeterminedminimum magnitude, and if the arterial pulsation occurs within thepredetermined time window (412), blood flow signal quality processor 72determines that the blood flow signal quality is acceptable (414),illuminates the blood flow quality indicator 12 green LED and enablesthe automatic measurement of LOP (416). If the blood flow signal doesnot meet any of the conditions described above, blood flow signalquality processor 72 determines that the blood flow signal quality isnot acceptable (418), illuminates the blood flow quality indicator 12red LED, and inhibits the automatic measurement of LOP (420). Anindication of blood flow signal quality is also provided on display 24.

It will be appreciated that additional criteria may be used by bloodflow signal quality processor 72 in evaluating signal quality; forexample, a force sensor could be integrated into blood flow transducer 2to measure the force applied to the patient's digit by blood flowtransducer 2. Signals from the force sensor could be used to furtheraugment the blood flow signal quality determination by providing anindication of when excess force is applied to the region of the digit towhich blood flow transducer 2 is applied. Excessive force applied byblood flow transducer 2 may lead to an unsuccessful or inaccurate LOPmeasurement due to reduced vascular blood flow.

Limb Occlusion Pressure Measurement

To automatically measure the limb occlusion pressure, controller 32 mustdetermine the minimum pressure required in cuff 10 to prevent arterialblood flow into patient limb 4 distal to the location of cuff 10. Asdescribed in detail below, controller 32 does this by analyzing signalsproduced by non-pulsatile level detector 68, by arterial pulsationdetector 70 and by blood flow signal processor 66 while increasing thepressure in cuff 10 to a pressure level at which arterial blood flow isno longer detectable above a minimum detection threshold.

To enable a better understanding of the sequence of operations completedand decisions made by controller 32 during the automatic measurement oflimb occlusion pressure a flow chart is provided in FIG. 5 and FIG. 6.

Referring to the flow chart in FIG. 5, when controller 32 detects thatLOP key 30 on keypad 26 or LOP key 14 on blood flow transducer 2 hasbeen depressed and the measurement of LOP has not been inhibited byblood flow signal quality processor 72 (502) it first determines if cuff10 is already inflated and being regulated as would be the case if theoperator of instrument had previously activated the inflate key onkeypad 26 (504). Controller 32 only responds to LOP key 30 or LOP key 14to initiate an LOP measurement sequence when cuff 10 is deflated andcontroller 32 is not regulating the gas pressure within cuff 10. Thissafety feature of the preferred embodiment prevents the operator frominadvertently initiating a measurement of LOP at a time when a surgicalprocedure may be in progress.

If controller 32 detects that LOP key 14 on blood flow transducer 2, orLOP key 30, or any other key on keypad 26 has been depressed while anLOP measurement sequence is in progress, controller 32 terminates theLOP measurement (506). An appropriate alarm message is shown on displaypanel 24 and controller 32 activates deflation valve 50 to vent gas fromcuff 10 (508). This allows the operator of instrument 6 to safely cancelan LOP measurement sequence that is in progress.

The LOP measurement sequence performed by controller 32 has two phases:an initialization phase during an initialization time period whenreference parameters are established; and a determination phase duringwhich the reference pressure level is monotonically increased until thepressure in cuff 10 reaches the limb occlusion pressure.

The initialization phase of the sequence for measuring LOP begins withcontroller 32 adjusting the intensity of IRLED 60 by communicating withconstant current source 64 (510). The intensity of IRLED 60 is set to alevel that produces a non-pulsatile photoplethysmographic signal at alevel indicated by non-pulsatile level detector 68 that is near apredetermined initial target level.

If the non-pulsatile photoplethysmographic signal cannot be set to alevel that is near the initial target level, such as may be the case ifblood flow transducer 2 is applied to a very thick digit or a to digitthat for other reasons absorbs a significant portion of the lightemitted by IRLED 60 (512), controller 32 determines that the LOPmeasurement sequence is unlikely to successfully complete, terminatesthe measurement attempt, and displays an appropriate message on displaypanel 24 (508). Also, if the amount of current that is required fromconstant current source 64 to produce a non-pulsatilephotoplethysmographic signal at a level near the initial target levelexceeds a predetermined maximum, then controller 32 also determines thatthe LOP measurement sequence is unlikely to successfully completebecause there will be insufficient adjustment range available to furtherincrease the intensity of IRLED 60 to compensate for changes in venousblood volume that may occur during the measurement.

Next, controller 32 sets the reference pressure level to a predeterminedinitial level of 30 mmHg (514). The pressure regulator then commencesinflation of cuff 10 to a pressure near 30 mmHg.

Controller 32 then waits for a predetermined maximum time period of 5seconds (516) for arterial pulsation detector 70 to detect threesequential blood flow pulsations with a magnitude greater than apredetermined minimum initial magnitude (518). If three sequentialpulsations that exceed the minimum initial magnitude are not detectedwithin the predetermined maximum time period, indicating that the LOPmeasurement attempt is unlikely to be successfully completed, then theLOP measurement sequence is terminated and the reference pressure levelis set to zero to start the deflation of cuff 10. A message is displayedon display panel 24 to alert the operator that an LOP measurement couldnot be completed (508), thus minimizing the duration of an LOPmeasurement attempt that might be unsuccessful and that might delay thestart of surgery and lead to excessive accumulation of venous blood.

If arterial pulsation detector 70 detects three sequential arterialblood flow pulsations that exceed the predetermined minimum initialmagnitude, controller 32 calculates the levels of reference parametersto be used in the determination phase of the LOP measurement sequence.Controller 32 chooses from the three sequentially detected pulsationsthe pulsation with the greatest magnitude (520), and the magnitude ofthis pulsation is selected by controller 32 as the reference magnitude.As described below, controller 32 makes comparisons of the magnitude ofsubsequent pulsations to the reference magnitude. Controller 32calculates a reference pulsation interval time which is the timeinterval between two of the three detected successive arterialpulsations (522). Controller 32 sets the refractory period of arterialpulsation detector 74 to 75 percent of the calculated referencepulsation interval time. Controller 32 also calculates the minimumdetection threshold and communicates this threshold to arterialpulsation detector 70. As described above, the minimum detectionthreshold determines the minimum magnitude of an arterial pulsation thatis detected by arterial pulsation detector 70. In the preferredembodiment, controller 32 computes the minimum detection threshold to bethe greater of 5 percent of the reference magnitude and a predeterminedminimum threshold.

Controller 32 next enters the determination phase of the LOP measurementsequence (524). The flow chart shown in FIG. 5 continues (526) in FIG. 6(602). FIG. 6 depicts the determination phase of the LOP measurementsequence; controller 32 begins by setting the reference pressure levelto a predetermined level of 95 mmHg (604). Controller 32 compensates forchanges in the amount of venous blood present in the volume of tissuebetween IRLED 60 and photodiode 62 that may occur during thedetermination phase of the LOP measurement sequence as follows. Eachtime an arterial blood flow pulsation is detected by arterial pulsationdetector 70 (606), controller 32 computes a new level for adjustableconstant source 64 and thereby the intensity of IRLED 60. Controller 32uses a proportional control algorithm to calculate a new level forconstant current source 64 that maintains the level of the non-pulsatilephotoplethysmographic signal from non-pulsatile level detector 68 nearthe target level set previously (608). The change to the intensity ofIRLED 60 is made during the refractory period of arterial pulsationdetector 70 so that artifacts that are caused by changing of theintensity of IRLED 60 do not affect arterial pulsation detector 70. Bycontinuously updating the level of constant current source 64 after eacharterial pulsation is detected in response to changes in thenon-pulsatile signal level, controller 32 can compensate for changes inthe absorption of light emitted by IRLED 60 due to changes in the amountof venous blood present in the volume of tissue illuminated by IRLED 60and maintain the non-pulsatile photoplethysmographic signal near thetarget level.

If during LOP measurement controller 32 detects that the level of thenon-pulsatile signal from non-pulsatile level detector 68 has exceeded apredetermined minimum or maximum limit level (610) controller 32terminates the LOP measurement and opens deflation valve 50 to deflatecuff 10 (612). Examples of conditions that may cause the non-pulsatilesignal to exceed the limits are the inadvertent removal of blood flowtransducer 2 from the digit during the measurement, an excessive amountof venous blood accumulating in the digit, failure of themulti-conductor cable 8, or failure of transducer 2. Controller 32 alsonotifies the operator by displaying an appropriate alarm message ondisplay panel 24 and by audio tones produced by speaker 54.

To increase the pressure in cuff 10 as rapidly as possible to the LOP,and at the same time to provide an accurate measurement of LOP,controller 32 operates as follows. Each time an arterial blood flowpulsation is detected by arterial pulsation detector 70 a new referencepressure level is calculated by controller 32. Near the time that thepulsation is detected, controller 32 records the level of the cuffpressure signal (614); this represents the pressure of gas in cuff 10near the time that the blood flow pulsation occurred. To ensure the cuffpressure signal accurately reflects the pressure of gas in theinflatable portion of cuff 10, pressure regulation is suspended (615)near the time that the blood flow pulsation occurs, as described above.Based on the magnitude of the detected blood flow pulsation incomparison with the reference magnitude an incremental pressure level iscalculated (616). Shortly after the detection of the blood flowpulsation and thus in synchrony with the pulsation, the referencepressure level is set by controller 32 to a level equal to the sum ofthe calculated incremental pressure level and the recorded cuff pressurelevel (618).

During the measurement of LOP, the magnitude of a detected arterialblood flow pulsation is dependent upon the pressure in cuff 10 at thetime the pulsation occurs. As the pressure in cuff 10 nears the pressurerequired to totally occlude arterial blood flow, the magnitudes ofarterial blood flow pulsations are reduced. To enable the preferredembodiment to rapidly increase the pressure in cuff 10 to the minimumpressure that occludes arterial blood flow, while not increasing thepressure in cuff 10 above that minimum pressure, the size of thepressure increment that is made after each detected arterial pulsationis dependent on the magnitude of the detected arterial blood flowpulsation. By making progressively smaller increments in pressure forcuff 10 as the cuff pressure nears the LOP, the preferred embodiment canmake a very rapid and accurate determination of LOP.

In the preferred embodiment, the incremental pressure level iscalculated as follows: 15 mmHg for a pulsation with a magnitude of 66percent of the reference magnitude or greater; 10 mmHg for a pulsationwith a magnitude of 50-65 percent of the reference magnitude; 7 mmHg fora pulsation with a magnitude of 33-49 percent of the referencemagnitude; 5 mmHg for a pulsation with a magnitude of 20-32 percent ofthe reference magnitude; and 3 mmHg for a pulsation with a magnitude ofless than 20 percent of the reference magnitude.

By making each increased reference pressure level equal to the sum ofthe calculated incremental pressure level that is based on the magnitudeof an arterial pulsation plus the recorded cuff pressure level (614) atthe time of that pulsation, and by increasing the reference pressurelevel in synchrony with that pulsation, the LOP measurement can proceedrapidly, accurately, and independently of the response timecharacteristic of the pressure regulator in combination with thepneumatic elements of the preferred embodiment. As an example, if thecuff pressure signal corresponds to a level of 133 mmHg when a pulsationis detected, and if the magnitude of the detected pulsation relative tothe reference magnitude is greater than 66 percent, then controller 32sets the reference pressure level to 148 mmHg (133+15) shortly after thepulsation. This is a more rapid and more accurate way to approach thetrue LOP in comparison to prior art apparatus in which each increasedreference pressure level is typically determined by adding apredetermined increment to the previous reference pressure level, and inwhich the reference pressure level is increased only after sufficienttime has elapsed to allow actual cuff pressure to reach the previousreference pressure level.

Referring again to FIG. 6, controller 32 continues to increase thereference pressure level each time a arterial blood flow pulsation isdetected by arterial pulsation detector 70 until an arterial blood flowpulsation is not detected for a period of time that is two times thereference pulsation to pulsation interval time determined during theinitialization phase of the LOP measurement sequence (620). When duringthe determination phase of the LOP measurement sequence an arterialblood flow pulsation is not detected for this period of time, controller32 calculates the limb occlusion pressure to be the pressure of gas incuff 10 as represented by the cuff pressure signal.

Controller 32 then deflates cuff 10 by setting the reference pressurelevel to zero and activating deflation valve 50 (622). Controller 32then calculates the recommended tourniquet pressure as described below(624) and displays the results of the LOP measurement on display panel24, this completes the LOP measurement sequence (626).

When the LOP has been determined controller 32 calculates a recommendedtourniquet pressure (RTP) by adding a predetermined offset pressurelevel (margin of safety) to the LOP. In the preferred embodiment theoffsets added to the LOP to calculate an RTP are consistent withrecommendations from the surgical literature and are calculated asfollows: if the LOP is greater than 190 mmHg the RTP is calculated byadding 100 mmHg to the LOP; if the LOP is greater than 130 mmHg the RTPis calculated by adding 75 mmHg to the LOP; or if the LOP is less that131 mmHg the RTP is calculated by adding 50 mmHg to the LOP.

Controller 32 displays the measured LOP and the calculated RTP ondisplay panel 24 and indicates that the measurement is complete. Forexample, if instrument 6 measures an LOP of 145 mmHg, then an RTP of 220mmHg is calculated and both the LOP and RTP are shown on display panel24. An operator may select the displayed RTP to be the referencepressure level or may manually select a different reference pressurelevel that is not based on LOP.

In the preferred embodiment, the difference in pressure between theselected reference pressure (RP), which may be the RTP, and the measuredLOP is defined as the Cuff Pressure Safety Margin (SM) {SM=RP−LOP}. TheCuff Pressure Safety Margin is automatically computed by controller 32and the computed value is shown on display panel 24 of instrument 6(628).

The systolic blood pressure of the patient is one factor that has beenshown to affect the measured LOP. Changes in blood pressure that occursubsequent to the measurement of LOP due to the effects of anesthesiaand other factors have an effect on the value of the Cuff PressureSafety Margin. To alert the operator to a change in the value of Cuffpressure Safety Margin instrument 6 computes and displays an UpdatedCuff Pressure Safety margin SM_(U)) when the patient's blood pressurecan be obtained from an external patient monitor by physiologiccharacteristic acquisition module 56 near the time of the LOPmeasurement and subsequently while cuff 10 is pressurized.

In the preferred embodiment, within a predetermined minimum time of thecompletion of an LOP measurement, the results of a non-invasive or realtime blood pressure measurement from an external patient monitor areacquired by physiologic characteristic acquisition module 56 (630) toestablish an LOP Reference Blood Pressure (BP_(LOPREF)). If the resultsof a recent blood pressure measurement are not available from anexternal patient monitor, controller 32 via physiologic characteristicacquisition module 56 may instruct the external patient monitor toperform a blood pressure measurement and obtain the resulting data toestablish a BP_(LOPREF).

To compute the value of the Updated Cuff Pressure Safety Margin whencuff 10 is pressurized to occlude blood flow subsequent to themeasurement of LOP, controller 32 automatically obtains the values ofrecent patient systolic blood pressure measurements (BP) via physiologiccharacteristic acquisition module 56. In the preferred embodiment, theUpdated Cuff Pressure Safety Margin is computed by the formula{SM_(U)=RP−LOP+k(BP_(LOPREF)−BP)}, where k is a constant value of 1.1.This relationship between Updated Cuff Pressure Safety Margin, bloodpressure and LOP has been chosen based on empirical data available atpresent. It will be appreciated that other mathematical relationshipsmay be used to determine the value of the Updated Cuff Pressure SafetyMargin.

The preferred embodiment includes means for the operator to increase ordecrease the reference pressure upon consideration of the displayedvalue of the Updated Cuff Pressure Safety Margin. The preferredembodiment also includes a Safety Margin Alarm Limit Window that may beautomatically set at predetermined levels above and below the CuffPressure Safety Margin by controller 32, or that may be set by theoperator of instrument 6 to desired levels above and below the CuffPressure Safety Margin. If the Updated Cuff Pressure Safety Marginexceeds the levels set by the Safety Margin Alarm Limit Window, theoperator of instrument 6 is alerted by an audible sound produced byspeaker 54 and by a message shown on display panel 24.

Typical Use in Surgery

To enable a better understanding of the preferred embodiment, itstypical use in a surgical procedure is described below.

An operator first selects an appropriately sized cuff 10 for applicationto patient limb 4 and secures cuff 10 around patient limb 4. Thepneumatic passageway from instrument 6 to the inflatable portion of cuff10 is completed by mating connectors 18 and 20. Many different sizes andshapes of cuff 10 may be optionally used with instrument 6 toaccommodate different physical sizes of patients and patient limbs.Cuffs may vary in length, width, shape, and application technique; alsosome cuffs may be applied with a soft limb protection sleeve locatedbetween the limb and the cuff. The specific level of pressure requiredin tourniquet cuff 10 to stop blood flow past cuff 10 at a particulartime is affected by variables including the characteristics of cuff 10and any underlying sleeve, the technique used in applying cuff 10, thephysiological characteristics of the patient, and the physicalcharacteristics of limb 4 at the location where cuff 10 is applied.

Accordingly, to assist in setting the reference pressure to the lowestand safest level, the operator of instrument 6 may choose to initiate ameasurement of LOP. To perform a rapid and accurate measurement of LOPthe operator first applies blood flow transducer 2 to a digit of patientlimb 4 distal to the position of cuff 10. Blood flow quality indicator12 may be used by the operator as a guide to indicate when blood flowtransducer 2 has been correctly applied to a digit of the patient's limband a blood flow signal is obtainable. If the quality of the blood flowsignal exceeds predetermined quality criteria as indicated by blood flowquality indicator 12, the operator may then initiate the measurement ofLOP by activating LOP key 30 on keypad 26 or LOP key 14 on blood flowtransducer 2. Instrument 6 then completes the LOP measurement within20-40 seconds as described above, by automatically increasing thepressure in cuff 10 to a pressure at which arterial blood flowpulsations can no longer be detected by blood flow transducer 2.Instrument 6 then displays the resulting LOP on display panel 24,together with the RTP, and then deflates cuff 10. For example an LOP of120 mmHg may be measured with a RTP of 170 mmHg. At this time,physiologic characteristic acquisition module 56 may obtain a patientblood pressure from an external patient monitor or initiate a bloodpressure measurement if the results of a recent blood pressure readingwere unavailable at the time of the LOP measurement.

The operator then selects a reference pressure level for the pressure ofgas to be maintained in cuff 10 during the surgical procedure. Theoperator may choose to accept the displayed RTP as the referencepressure level or the operator may manually set another referencepressure level based on his or her judgment, experience or theinstitutional protocol. A Cuff Pressure Safety Margin is then computedbased on the measured LOP and selected reference pressure and shown ondisplay panel 34.

For example, an LOP of 120 mmHg is measured, the corresponding RTP of170 mmHg is displayed, and the operator selects a reference pressure of180 mmHg for a Cuff Pressure Safety Margin of 60 mmHg. The patient'sblood pressure near the time of LOP measurement (BP_(LOPREF)) is 90mmHg.

The subsequent inflation of cuff 10 to a pressure near the selectedreference pressure level is then initiated by the operator depressingthe “inflate” key on keypad 26. The pressure regulator of instrument 6then operates to maintain the pressure of gas within cuff 10 near theselected reference pressure level. The reference pressure level may beadjusted and set to a new level at any time by the operator ofinstrument 6. Physiologic characteristic acquisition module 56 ofinstrument 6 operated to acquire values of monitored physiologiccharacteristics throughout the surgical procedure. Controller 32computes an Updated Cuff Pressure Safety Margin for display andgenerates alarms if the Updated Cuff Pressure Safety Margin exceedsalarm limits. Referring to the example above, at time during the surgerythe patient's blood pressure increases to 130 mmHg due to variations inanesthesia and physiologic response to surgery. An Updated Cuff PressureSafety Margin of 16 mmHg is computed and displayed{SM_(U)=180−120+1.1(90−130)}, the operator is alerted to the change inSafety Margin by an alarm and adjusts the reference pressure to 220 mmHgresulting in an Updated Cuff Pressure Safety Margin of 56 mm Hg.

At the completion of the surgical procedure, the operator initiates thedeflation of cuff 10 by activating the deflate key on keypad 26. Cuff 10is then removed from patient limb 4 immediately after deflation. Cuff 10may be disconnected from instrument 6 by releasing connectors 18 and 20.

The invention claimed is:
 1. A method of determining for a patient anocclusion pressure of a limb to which an inflatable and deflatable cuffis attached to encircle the limb; comprising steps of: connecting to thecuff a tourniquet instrument that includes a sensor for sensing a levelof pressure in the cuff; regulating pressure in the cuff by directingthrough a port in the cuff a flow of fluid into and out of the cuff;detecting an arterial pulsation of blood flow past the cuff andproducing a blood flow signal indicative of the blood flow; using acontroller for responding to the detection of the arterial pulsation bysuspending regulation of the pressure in the cuff for a measurement timeperiod; and for sensing a level of pressure in the cuff during themeasurement time period thereby to provide a cuff pressure signalindicative of the sensed level of pressure in the cuff during themeasurement time period; and for increasing the cuff pressure level inresponse to the blood flow signal and cuff pressure signal until bloodflow indicated by the blood flow signal decreases to a level less than aminimum detection threshold thereby to produce a limb occlusion pressurevalue that is a based upon pressure in the cuff sensed during themeasurement time period.
 2. The method of claim 1 wherein suspendingregulation of the pressure in the cuff includes closing valves thatcontrol the flow of fluid into and out of the cuff.
 3. The method ofclaim 1 including steps of calculating a recommended tourniquet pressureas the sum of an initial limb occlusion pressure value and a safetymargin level and occasionally updating the recommended tourniquetpressure level based upon changes in the patient's physiology.
 4. Themethod of claim 3 including a step of producing an alarm signal when theupdated recommended tourniquet pressure level is outside of apredetermined range.
 5. The method of claim 1 including steps ofattaching a blood flow transducer to a portion of the limb and equippingthe transducer with an indicator for producing two signals when apredetermined minimal amount of blood is and is not, respectively,flowing in the portion of the limb.
 6. The method of claim 5 including astep of providing the controller in the tourniquet instrument, themethod further comprising a step of equipping the transducer with a keythat is operable to signal the controller to operate.
 7. The method ofclaim 6 including a step of enabling and disabling the operation of thekey depending upon the signal produced by the indicator.
 8. The methodof claim 5 including a step of initializing the blood flow transducer toconfirm the transducer is attached to the limb portion.
 9. The method ofclaim 5 including a step of initializing the blood flow transducer toconfirm the transducer is unaffected by ambient light above apredetermined threshold level.
 10. The method of claim 1 including astep of establishing the measurement time period to be a specific timeduration.
 11. The method of claim 10 wherein the establishing stepincludes establishing the measurement time period to be about 30milliseconds.